The present invention relates to a copper contact jaw which is insulated from and releasably fastened to the electrode mount of an electric furnace for the transfer of current from the input leads to the electrode.
In the past, such contact jaws have been cast in one or more parts or have been welded and possibly provided with a stainless steel reinforcement. An insulating layer has been disposed between the contact jaw and the electrode mount to prevent the generation of secondary currents. Since this insulating layer is subject to damage, so that frequent exchange of the insulating layer becomes necessary, it has already been disclosed to overcome this drawback by covering the faces of the electrode mount opposite the contact jaw with a coating of aluminum oxide. However, the thickness of such a coating was found to be insufficient, so that neither a sufficient insulating effect nor a satisfactory service life was realized for the insulating layer.
SUMMARY OF THE INVENTION
It is an object of the present invention to overcome this drawback and to provide a contact jaw of the abovementioned type which is able to withstand very high stresses. This object can be attained by providing a hard copper contact jaw which, at least where it faces the electrode mount, is provided with a metal adhesion layer and a coating of ceramic insulating material on the adhesion layer.
Compared to prior art contact jaws, the contact jaw according to the present invention has the advantage that the layer thickness of the insulating coating can be increased considerably. It is advantageous to laterally support the coating on the contact jaw at a pair of guides disposed at the electrode mount.
Particularly advantageous results are attained if a nickel-aluminum alloy, whose expandability results in good adhesion, is used as the metal adhesion or bonding layer.
The coating of ceramic insulating material is preferably pure aluminum oxide. It is of particular advantage if the coating is applied by a spray process. Particularly great thickness of the coating can be produced if the contact jaw is made out of a chromium-zirconium alloyed and heat treated hard copper in which the zirconium content lies between 0.003 and 0.5% and the chromium content lies between 0.3 and 1.2%. These contents depend, in particular, on the homogeneity of the copper melt, i.e. with great homogeneity the lower limit values may be sufficient. A heat treatment to temper the material before the insulating coating is applied results in an even greater improvement of the mechanical values of the hard copper.